A DC-DC converter generates an output direct-current voltage based on an input direct-current voltage. Such a DC-DC converter suffers increased power loss in the internal circuits thereof when a voltage difference between the input direct-current voltage and the output direct-current voltage increases.
FIG. 1 is a drawing illustrating an example of the loss characteristics of a DC-DC converter. Specifically, the example of loss characteristics illustrated in FIG. 1 is the characteristics of Texas Instruments TPS61200 converters. The horizontal axis represents input voltage, and the vertical axis represents efficiency. A 5-V-type DC-DC converter that generates an output direct-current voltage of 5 V exhibits an efficiency curve 10, and a 3.3-V-type DC-DC converter that generates an output direct-current voltage of 3.3 V exhibits an efficiency curve 11. In the case of the input direct-current voltage being 5 V, the 5-V-type DC-DC converter exhibits an efficiency of approximately 90% (i.e., 10% loss), and the 3.3-V-type DC-DC converter exhibits an efficiency of approximately 60% (i.e., 40% loss). In the case of the input direct-current voltage being 3.3 V, the 5-V-type DC-DC converter exhibits an efficiency of approximately 55% (i.e., 45% loss), and the 3.3-V-type DC-DC converter exhibits an efficiency of approximately 80% (i.e., 20% loss).
In an electronic apparatus that uses a rechargeable battery, a DC-DC converter inside the electronic apparatus converts the voltage of the rechargeable battery into a predetermined power supply voltage, which is then supplied to the circuits inside the electronic apparatus. In such a configuration, the output voltage of the rechargeable battery changes in response to the battery level of the rechargeable battery, which results in changes with time in the input direct-current voltage of the DC-DC converter.
Recently, a technology called “energy harvest” has been proposed that acquires electric power from environment and stores the acquired power in a capacitor for subsequent use. In this energy harvest, the charge level of the capacitor constantly changes due to the unstable nature of power supply extracted from environment. The use of energy harvest as the power supply source for an electronic apparatus thus gives rise to a problem of constant changes in the input direct-current voltage of the DC-DC converter.
A change in the input direct-current voltage of a DC-DC converter causes an actual input direct-current voltage to divert from the input direct-current voltage that ensures minimum power loss in the DC-DC converter, thereby resulting in a situation in which the electronic apparatus is operating in a condition that creates significant power loss.    [Patent Document 1] Japanese Patent Application Publication No. 2010-213466